1. Field of the Invention
The present invention generally relates to optical amplifier systems and, more particularly, relates to optical filters for flattening the gain of an optical amplifier over an operating wavelength band.
2. Technical Background
In an optical network, optical signals are typically transmitted through a fiber over relatively long distances. Because the strength of the optical signals tends to decrease with increasing transmission path length, it has become commonplace to divide the fibers into spans, with in-line optical amplifiers positioned between the spans. The typical span is, for example, 80-120 km in length. While the in-line optical amplifiers boost the signal strength of the transmitted optical signals, such optical amplifiers typically do not exhibit flat gain characteristics over the band of wavelengths of the optical signals that are transmitted through the optical amplifier. Thus, in an optical network, if each of the optical amplifiers positioned between each fiber span amplify optical signals having certain wavelengths more than they amplify optical signals having other wavelengths, some optical signals will not be amplified sufficiently over a long transmission path and those signals will be more susceptible to errors.
To provide for uniformity of signal amplification at each span of all optical signals transmitted through the network, various techniques have been proposed to flatten the gain of the optical amplifiers so that all the optical signals are amplified the same amount by each optical amplifier provided along a given transmission path. One technique that has been proposed is to provide a gain-flattening dielectric optical filter that has an insertion loss spectrum (also referred to as the xe2x80x9ctransmission spectrumxe2x80x9d) that is inversely related to the gain spectrum of the optical amplifier. In other words, the gain-flattening filter will attenuate those wavelengths that are more greatly amplified by the optical amplifier such that the output of the gain-flattened amplifier exhibits a substantially flat and equal gain for all the wavelengths in the wavelength band of interest.
One problem that arises through the use of optical amplifiers and, in particular, erbium-doped fiber amplifiers, is that the gain spectrum tends to vary with fluctuations in the operating temperature. Such optical amplifiers may be exposed to operating temperatures ranging from xe2x88x925xc2x0 C. to 75xc2x0 C. Unless otherwise compensated, the gain spectrum of the optical amplifier will change significantly with temperature. In general, all components in the amplifier contribute to this variation, but the dominant contributions are typically from the gain medium (i.e., the erbium-doped fiber coil).
Some techniques that have been employed to compensate for these thermal gain variations, which are also known as xe2x80x9cthermal wigglexe2x80x9d or xe2x80x9cthermal ripple,xe2x80x9d include providing a heating system for maintaining the erbium-doped fiber coil of the optical amplifier at temperatures close to the upper range of the operating temperatures to which the optical amplifier would otherwise be exposed or to provide, and providing a thermal controller for double-sided control whereby the fiber coil is maintained at a fixed temperature by heating and cooling. Thus, these solutions basically eliminates the cause of the variation in the gain spectrum of the optical amplifier medium.
By providing a heating system in the optical amplifier and thereby stabilizing the gain spectrum with respect to temperature, designers of optical amplifier systems have made concerted efforts to design gain-flattening filters whose insertion loss spectrums do not vary as a function of operating temperature. By stabilizing both the gain spectrum of the optical amplifier and the insertion loss spectrum of the gain-flattening filter with respect to temperature, the overall gain of the optical amplifier system remains substantially flat over the wavelength band of interest (i.e., generally 1530-1560 nm) and maintains the substantially flat characteristic over the operating temperature range.
Although the above-described solution has become commonplace, the use of a heating system introduces several disadvantages. First, the heating system itself adds considerable expense not only in terms of manufacturing and installation costs, but it also adds significantly to the thermal budget for removing heat from the entire module and adds a reliability risk if the thermal control system should fail.
A solution to these problems is disclosed in U.S. Pat. No. 6,049,414 issued to Espindola et al. (the ""414 patent) The ""414 patent discloses a gain-flattening filter having a transmission spectrum that varies as a function of temperature so as to compensate for variations in the gain spectrum of the optical amplifier that occur as a function of wavelength and temperature. The gain flattening filter disclosed in the ""414 patent includes a composite filter having a plurality of concatenated fiber Bragg gratings (FBG) or a long period gratings each having different shift coefficients. By providing such a gain flattening filter, the need for a thermal controller is effectively eliminated.
While the solution disclosed in the ""414 patent is sound in theory, the design and construction of such a gain flattening filter is not practical due to the difficulty in simply designing a FBG or long fiber grating filter that compensates for amplifier gain as a function of wavelength, let alone designing a FBG or long fiber grating filter that also compensates for amplifier gain as a function of temperature.
In view of the above-described problems associated with providing a heating system for each optical amplifier provided in an optical network, and the problems associated with designing a practical gain flattening filter that also compensates for thermal wiggle, there exists a need for an optical amplifier system that does not require the use of a heater or thermal control system, has a gain spectrum that does not vary significantly as a function of operating temperature, and is practical to design and construct.
An aspect of the present invention is to provide an optical device comprising an optical amplifier to amplify optical signals received through an optical input and to supply the amplified optical signals from an optical output. The optical device further comprises an optical filter component to compensate for variations in the gain spectrum of the optical amplifier that occur as a function of wavelength and operating temperature. The optical filter component includes a first optical filter having an athermalized transmission spectrum, and a second optical filter having a transmission spectrum that varies as a function of operating temperature.
Another aspect of the present invention is to provide an optical filter for flattening the gain of an optical amplifier that has a gain spectrum that varies as a function of operating temperature. The optical filter comprises a first optical filter portion having an athermalized transmission spectrum, and a second optical filter portion having a transmission spectrum that varies as a function of operating temperature. The first and second optical filter portions combine to compensate for variations in the gain spectrum of the optical amplifier that occur as a function of wavelength and operating temperature.
It is another aspect of the present invention to provide a method of compensating for variations in the gain spectrum of an optical amplifier that occur with variations in operating temperature. The method comprising the steps of (a) providing a first optical filter having an athermalized insertion loss spectrum, and (b) providing a second optical filter having an insertion loss spectrum that varies with fluctuations in operating temperature, and that, when combined with the insertion loss spectrum of the first optical filter, compensates for variations in the optical amplifier gain as a function of wavelength.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.